UNDERGROUND RAILWAYS
The underground railway is the quickest, safest, most reliable and comfortable means of city's transport. Metro can solve the problems of carrying a great number of passengers in large cities as well as the problems of traffic jams, air contamination and noise.
However, from the technical point of view the underground railway system is very expensive and complicated construction work. This system includes subsurface lines, ground-based lines and elevated lines. Ground-based lines are usually used at the terminals of the underground railways. Besides the underground and surface structures involve stations, tunnels, escalators, underpasses, ventilation and sanitary engineering as well as a power supply system.
The permanent way of underground railways differs from the usual railway track. The sleepers are only 0.9 m long. They are shorter than those of the railroad track which are 2.7 m long. The sleepers of the usual track are laid upon ballast made of broken stone or other materials. The crossties of the underground railway are laid directly on concrete base. It is more expensive but keeps air free from dust. If the ballast were made of slag, gravel, sand or even broken stone, the train would be followed by the dust clouds.
All Metro trains are powered by electricity. The current is obtained from the third rail. This contact rail is laid along the whole track and transmits the direct current of 825 volts to the train electric motor through the pantograph.
The Underground's carrying capacity depends on the number of coaches, which ranges from 2 to 8 per train, and the frequency of train running, which ranges from 80 seconds to 8 minutes.
Nowadays there are underground railways in 80 cities all over the world. The first Metro railway lines were constructed in London in 1863. It is the oldest but not the longest underground. New York was the second city to build the subway. The first track was laid in 1868 and nowadays it is the longest Metro in the world. The shortest Metro line was constructed in Turkey. Its length is only 600 m but Istanbul is very proud of the metropolitan means of transport.

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